We are investigating cell cycle checkpoint activation mediated by the DNA mismatch repair proteins MutSalpha and MutLalpha in response to several DNA damaging agents including SN1 alkylators, fluorouracil, and cisplatin. The cellular response to DNA damage involves the activation of signaling kinases such as ATR and ATM. This, in turn, activates a cascade of signaling kinases including Chk1 and Chk2 that ultimately result in cell cycle arrest in S phase and at the G2/M boundary. We are purifying recombinant human proteins involved in this damage response with the goal of defining the underlying molecular mechanism. We also seek to identify sites of mismatch repair in the human genome. We are also examining the role of nuclear membrane proteins in coordinating dynamic changes in nuclear architecture during the cell cycle and in response to DNA damage. We are identifying sites of cell cycle-dependent post-translational modification of these proteins and are using a variety of functional assays to define the roles that these modifications play in response to signals for cell division and DNA damage.